Methanol synthesis

ABSTRACT

Methanol is synthesised in a synthesis loop wherein recycled unreacted gas, optionally together with part of the make-up gas, is passed through a bed of synthesis catalyst under methanol synthesis conditions, make-up gas is then added and the mixture passed through at least one further bed of synthesis catalyst under methanol synthesis conditions prior to separation of the synthesised methanol. Preferably the further bed of synthesis catalyst is located in a heat exchange reactor producing pressurised hot water which is employed to saturate a hydrocarbon feedstock from which the make-up gas is produced by steam reforming.

This application is a continuation of PCT/GB99/01334 filed Apr. 29,1999.

This invention relates to methanol synthesis. Methanol is conventionallysynthesised at elevated and pressure in a methanol synthesis loop wheresynthesis gas, containing hydrogen, carbon oxides, and, usually, someinerts such as nitrogen and methane, is passed over a copper catalyst atan elevated temperature, typically 200-300° C., and pressure, typically40-150 bar abs., and then the product reacted gas is cooled, condensedmethanol is separated and the unreacted gas is recycled to the synthesisreactor. Fresh synthesis gas, hereinafter termed make-up gas, is addedto the loop at a suitable location, usually to the recycled unreactedgas before the latter is fed to the synthesis reactor. A purge is takenfrom the loop at a suitable point to avoid the build-up of inerts to anuneconomically high level. The make-up gas may be added to the loopbefore or after the separation step.

Methanol synthesis is an exothermic process and it is necessary to limitthe amount of reaction occurring in a bed of catalyst and/or to cool thebed, to avoid overheating the catalyst. To this end, a variety ofreactor types have been employed. For example it has been proposed toemploy a reactor with means to inject cool quench gas (generally amixture of make-up gas and unreacted recycle gas) into the catalyst bedor between beds. Examples of such quench bed reactors are described inGB 1105614, EP 0297474, EP 0359952 and U.S. Pat. No. 4,859,425. It hasalso been proposed to employ reactors having heat exchangers within thebeds so that heat evolved by the reaction is transferred to a coolant.Thus in the arrangement described in U.S. Pat. No. 4,778,662 thesynthesis reactor has coolant tubes which extend through at least theinlet part of the catalyst bed and open into the space above the inletto the catalyst bed: the coolant is the mixture of recycled unreactedgas and make-up gas so that the reactants are heated to the desiredinlet temperature by the evolved heat. In the arrangement described inGB 2046618 the catalyst is disposed as a single bed through which thereactants flow radially and heat exchange tubes are provided throughwhich a coolant, e.g. pressurised boiling water, is circulated.

It is often desirable to increase the amount of methanol synthesised. InU.S. Pat. No. 5,252,609 and U.S. Pat. No. 5,631,302 methods aredescribed wherein the make-up gas is subjected to a preliminarysynthesis step before it is added to the synthesis loop. In theaforesaid U.S. Pat. No. 5,631,302 the second synthesis stage, i.e. thatin the synthesis loop, is effected in heat exchange with boiling water,thereby producing steam which may be exported. In EP 0790226 anarrangement is described where there are two synthesis reactors inseries in the loop; the first reactor being cooled by heat exchange withboiling water while the second is cooled by heat exchange with themixture of make-up gas and recycled unreacted gas.

In the aforementioned arrangements wherein the coolant is boiling water,the reactor operates under essentially isothermal conditions and thetemperature and pressure of the steam produced is largely dependent uponthe temperature at which the reactants leave the synthesis reactor. Inorder to achieve a high conversion per pass, this temperature isdesirably relatively low, for example in the range 200 to 250° C. As aresult the temperature and pressure of the steam is such that the steamis of little utility elsewhere in the methanol plant.

The make-up gas is often produced by a steam reforming process wherein ahydrocarbon feedstock, such as natural gas, is reacted with steam at anelevated pressure, e.g. in the range 20 to 80 bar abs., and at anelevated temperature, e.g. in the range 700 to 1100° C., in the presenceof a catalyst. This reforming reaction is strongly endothermic and atleast part of the reforming reaction is generally operated with thecatalyst disposed in tubes through which the feedstock/steam mixturepasses while the tubes are heated externally by a suitable medium.

It is known, e.g. see U.S. Pat. No. 4,072,625, to recover heat fromreacted methanol synthesis gas leaving a methanol synthesis reactor byheat exchange with water under pressure to give a stream of heated waterwhich is used to provide at least some of the steam required for steamreforming by contacting the stream of hot water, preferably afterfurther heating, directly with the hydrocarbon feedstock. Such directcontact of the hydrocarbon feedstock with hot water is herein termedsaturation. We have realised that. instead of recovering the heat fromthe reacted synthesis gas after it has left the synthesis reactor, byusing a reactor operated in heat exchange with water under such apressure that the water does not boil, hot water useful for saturationcan be obtained, while at the same time enabling an adequate temperatureprofile to be achieved in the synthesis reactor.

In the present invention, the synthesis loop comprises two or moresynthesis stages in series with at least the final stage being effectedin indirect heat exchange with water under sufficient pressure toprevent boiling, and the resultant heated pressurised water is used tosupply at least some of the process steam required for the aforesaidreforming reaction by contacting the hydrocarbon feedstock with thepressurised heated water. It will be appreciated that since the water iscontacted directly with the hydrocarbon feedstock, the pressure of thepressurised water is equal to or greater than that employed in thereforming reaction.

According to the present invention we provide a process wherein methanolis synthesised in a loop from a synthesis gas mixture comprisinghydrogen and carbon oxides in at least two synthesis stages, synthesisedmethanol is separated, at least part of the unreacted synthesis gas isrecycled to the first stage, and make-up gas is added to the loop,characterised in that in at least the final synthesis stage of the loop,the synthesis is effected in indirect heat exchange with water undersufficient pressure to prevent boiling, whereby a stream of heatedpressurised water is produced, and the make-up gas is produced by aprocess including catalytically reacting a hydrocarbon feedstock withsteam at an elevated temperature and at an elevated pressure equal to orless than the pressure of said stream of heated pressurised water and atleast part of said steam is introduced by direct contact of saidhydrocarbon feedstock with said stream of heated pressurised water.

In contrast to the process of the aforesaid EP 0790226 where the firststage is effected in indirect heat exchange with boiling water, in thepresent invention at least the final stage is effected in heat exchangewith water under sufficient pressure to prevent boiling. The reactorused for synthesis in indirect heat exchange with pressurised water isherein termed a water-cooled reactor.

In its simplest form the synthesis loop has two stages of methanolsynthesis with one or both stages being effected in a water-cooledreactor. The first stage is preferably effected in a quench reactor or aheat exchange reactor wherein the synthesis catalyst is cooled bytransferring heat evolved by the synthesis reaction by heat exchange tothe feed gas of that reactor, e.g. as described in the aforesaid U.S.Pat. No. 4,778,662. Where more than two stages are employed, it is againpreferred that the first stage is effected in a quench reactor or a heatexchange reactor as aforesaid and at least the last of the subsequentstage or stages is effected in the water-cooled reactor.

Where, as is preferred, the first synthesis stage is not effected in awater-cooled reactor, it may be desirable that at least part of themake-up gas is added to the loop after the synthesis gas has beensubjected to the first synthesis stage and before it is subjected to thesynthesis stage employing the water-cooled reactor. One advantage ofthis arrangement is that the throughput may also be increased byoperating the loop at a lower circulation ratio, which is defined hereinas the ratio of the flow rate of the gas recycled from the separator tothe rate at which make-up gas is fed to the loop. In a conventionalmethanol synthesis process, this circulation ratio is generally in therange 3 to 7. By adding at least part of the make-up gas after the firstsynthesis stage, low circulation ratios, e.g. in the range 1 to 4,particularly 1 to 3, may be employed. The addition of part of themake-up gas after the first synthesis stage is of particular benefit atcirculation rates below 2.5, especially below 2. If, in a loop operatingat a low circulation rate, all the make-up gas is added to the recycledunreacted gas before the first synthesis stage, the partial pressures ofthe reactants of the gas fed to the first stage may be relatively highleading to excessive reaction, and heat evolution, in the first stage.

It is preferred that at least 5% of the make-up gas is added to therecycled unreacted gas before the latter is fed to the first synthesisstage. While all of the make-up gas may be added to the recycledunreacted gas before the latter is fed to the first synthesis stage, itis preferred that at least 10%, particularly at least 30%, of themake-up gas is added to the loop after the first synthesis stage,especially if the circulation rate is low, e.g. below 2. The proportionof the make-up gas that is added to the loop after the first synthesisstage will depend upon the type of reactor employed for the firstsynthesis stage and on the circulation ratio.

The first synthesis stage is preferably effected adiabatically.

Thus in one form of the invention, the first stage employs a quenchreactor wherein some or all of the recycled unreacted gas, optionally towhich part of the make-up gas has been added, is fed to the inlet andsome or all of the remainder of the make-up gas, or make-up gas inadmixture with recycled unreacted gas, is used as the quench gas. Thegas from the outlet of the quench reactor is then fed to thewater-cooled reactor.

Where a quench reactor is employed for the first synthesis stage,typically only about 20-25% of the recycled unreacted gas is fed to thequench reactor inlet: the balance, to which make-up gas may be added, isused as the quench gas. The quench reactor may have several beds ofsynthesis catalyst with injection of quench gas between each bed. Withsuch a reactor it is preferred that at least 50% of the make-up gas isadded as some or all of the quench gas and/or to the reacted gas fromthe quench reactor after the first synthesis stage, i.e. before it isfed to the water-cooled reactor.

Where a heat exchange reactor, e.g. of the type described in U.S. Pat.No. 4,778,662, wherein the catalyst is cooled by transferring heatevolved by the synthesis reaction by heat exchange to the feed gas tothat reactor, is employed for the first stage, a larger proportion, forexample 30 to 90%, particularly 40 to 70%, of the make-up gas may beadded to the recycled unreacted gas before the latter is fed to thefirst synthesis stage. Indeed, unless operating at very low circulationrates, e.g. below 2, all the make-up gas may be added to the recycledunreacted gas before the latter is fed to the first synthesis stage.After leaving the first synthesis stage, the remainder, if any, of themake-up gas is added and the mixture passed through one or more furthercatalyst beds, disposed in the water-cooled reactor.

The water-cooled reactor may have the catalyst disposed in tubes withthe pressurised water circulating past the exterior of the tubes.However it is preferred that the catalyst is disposed as a single bedwith the pressurised water passing through cooling tubes disposed withinthe catalyst bed.

In the present invention, the heated pressurised water is employed tosupply at least part of the steam required for making the make-up gas.Thus the heated pressurised water, preferably after further heating, isdirectly contacted with the hydrocarbon feedstock before the latter issubjected to the reforming reaction. Such direct contact of thehydrocarbon feedstock with hot water is herein termed saturation. Itwill be appreciated that since the water is contacted directly with thehydrocarbon feedstock, the pressure of the pressurised water is equal toor greater than that employed in the reforming reaction. Normally, thefeedstock, e.g. natural gas, at an elevated pressure is subjected todesulphurisation prior to reforming. It is generally desirable to effectthe contacting with the pressurised water after any suchdesulphurisation step.

In a preferred arrangement, the reforming is effected in two stages. Inthe first, primary reforming, stage the feedstock/steam mixture ispassed over a steam reforming catalyst, usually nickel supported on aninert support, e.g. alumina or a calcium aluminate cement, disposed inexternally heated tubes. In the second stage, the primary reformed gasmixture is subjected to a secondary reforming stage wherein it ispartially combusted with oxygen and passed through a secondary reformingcatalyst. The secondary reforming catalyst is normally disposed as asingle bed, again usually of nickel supported on an inert support, e.g.alumina or a calcium aluminate cement. By adjusting the amount of oxygenemployed relative to the amount of feedstock, a secondary reformed gasthat approximates to the stoichiometric composition for methanolsynthesis may be obtained. If the secondary reforming stage is omitted,the reformed gas is liable to have an excess of hydrogen over thatrequired for methanol synthesis, especially where the feedstock isnatural gas. In a preferred version of a reforming process employingprimary and secondary reforming, the primary reforming is effected in aheat exchange reformer with the heating required for the primaryreforming stage being provided by passing the secondary reformed gaspast the tubes containing the primary reforming catalyst.

The reformed gas is cooled and excess steam condensed therefrom beforecompression, if any, of the reformed gas to the synthesis loop pressure.The cooling of the reformed gas preferably includes further heating ofthe pressurised water before the latter is contacted with thehydrocarbon feedstock. It may also include other heat recovery, e.g.heating of pressurised water fed to the synthesis reactor, and theprovision of heat for distillation of product methanol.

The invention is illustrated by reference to the accompanying drawingswherein

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowsheet of one embodiment of the invention;

FIG. 2 is a flowsheet of an alternative methanol loop arrangement foruse in the flowsheet of FIG. 1;

Referring to FIG. 1, a hydrocarbon feedstock, such as natural gas, atelevated pressure, e.g. 45 bar abs., is fed as stream A via line 10,mixed with a hydrogen-containing gas 11 (stream B), and fed to heatexchangers 12 and 13 wherein it is heated to a temperature suitable fordesulphurisation. The gas is passed through a bed of ahydrodesulphurisation catalyst, e.g. nickel and/or cobalt molybdate, anda bed of a sulphur absorbent, e.g. zinc oxide, in vessel 14 to effectdesulphurisation of the gas. The desuiphurised gas serves as the heatingmedium in heat exchanger 12 and then is passed as stream C to asaturator 15. In saturator 15, the desulphurised feedstock is contactedwith heated water, at a pressure similar to that of the desulphurisedfeedstock, fed as stream D via line 16. The saturated feedstock, i.e. afeedstock/steam mixture, is then fed via line 17 to a heater 18 where itis further heated and then fed via line 19 as stream E to a heatexchange reformer 20. Heat exchange reformer 20 has a plurality of tubes21 containing a steam reforming catalyst, e.g. nickel supported on acalcium aluminate cement rings. The reformer tubes 21 are heated by ahot gas flowing through the shell space 22 of the heat exchange reformer20. The feedstock/steam mixture undergoes primary reforming in the tubes21 and is then fed as stream F via line 23 to a secondary reformer 24.In secondary reformer 24, the primary reformed gas is partiallycombusted with oxygen fed as stream G via line 25 and the partiallycombusted mixture is fed through a bed 26 of a secondary reformingcatalyst, e.g. nickel supported on a support of calcium aluminatecement, where it undergoes secondary reforming. The resultant hot gas,comprising hydrogen and carbon oxides plus unreacted steam and a littleunreacted methane, is then fed as stream H via line 27 to the shellspace 22 of the heat exchange reformer wherein it serves to heat thereformer tubes 21. The partially cooled secondary reformed gas leavesthe heat exchange reformer 20 as stream I via line 28. The secondaryreformed gas is then further cooled by heat exchange in heat exchangers29, 30 and 31 to below the dew point of the steam in the secondaryreformed gas. The unreacted steam thus condenses and is separated asstream J from the secondary reformed gas in a separator 32. Theresultant dewatered gas is then compressed in compressor 33 to formfresh synthesis gas, i.e. the make-up gas, (stream K) at about thedesired synthesis pressure. The make-up gas is fed to a synthesis loopvia lines 34 (stream L) and/or 35 (stream M).

In the synthesis loop, any make-up gas that is fed via line 35 is mixedwith recycled unreacted gas supplied as stream N via line 36 from acirculator 37. The resultant mixture, stream 0, is then fed via line 38to a heat exchange reactor 39. The synthesis gas passes up through tubes40 surrounded by a bed 41 of methanol synthesis catalyst. The synthesiscatalyst is typically the product of reducing to copper metal the copperoxide in a catalyst precursor containing oxides of copper, and othermetals such as zinc. chromium, aluminium, magnesium and/or rare earths.Copper/zinc oxide/alumina catalysts are preferably employed. As the gaspasses up through tubes 40 it is heated to the desired synthesis inlettemperature, which is typically in the range 200 to 240° C., and thenpasses down through the bed of synthesis catalyst. Methanol synthesisoccurs with heat evolved heating the incoming gas passing up throughtubes 40. The resultant reacted gas, comprising methanol and unreactedgas, is then passed as stream P via line 42 to a water-cooled reactor43.

Make-up gas may be supplied via line 34 as stream L and added to themixture to give stream Q before it enters water-cooled reactor 43. Inreactor 43, the partially reacted synthesis gas passes through a bed 44of methanol synthesis catalyst through which pass a plurality of tubes45 through which water at a pressure substantially equal to thereforming pressure, e.g. 45 bar abs., is passed as a coolant. Moremethanol synthesis occurs as the gas passes through the bed 44 with theheat evolved heating the pressurised water. The reacted gas leaves thewater-cooled reactor 44 as stream R via line 46 and is cooled, to belowthe dew point of the methanol therein, in heat exchanger 47. Thecondenses crude methanol is separated in separator 48 and is collectedas stream S via line 49. The crude methanol may then be subjected todistillation as is well known in the art.

The unreacted gas from which the crude methanol has been separated isrecycled as stream T via line 50 to the circulator 37. Part of theunreacted gas is taken via line 51 as a purge stream U; part of thepurge is fed as the hydrogen-containing gas fed via line 11 as stream Bwhile the remainder is purged via line 52 and used as fuel, e.g. it maybe combusted and the combustion products used to heat heat exchanger 18.The hot pressurised water leaves water-cooled reactor 43 via line 53 andis further heated in heat exchanger 29 to provide the hot pressurisedwater stream D fed to the saturator 15 via line 16. In some cases it maybe necessary to heat the hot pressurised water from heat exchanger 29further in a heat exchanger 54 which may also be heated by the purge gascombustion products. The surplus water from the saturator 15 is drainedvia line 55. Part of the surplus water is discharged via line 56 asstream V. To the remainder make-up water is added as stream W via line57 and the mixture heated in heat exchanger 30 and returned to thewater-cooled reactor 43 via line 58.

In some cases it may be desirable to increase the temperature of thecoolant water entering the water-cooled reactor 43 via line 58 byrecycling part of the hot pressurised water leaving the reactor 43 vialine 53 directly back to line 58 as stream X via the line 59 showndotted in FIG. 1 so that the coolant stream Y fed to the watercooledreactor 43 is a mixture of stream X and the water supplied via line 58.This may be desirable to prevent overcooling of the reactants inwater-cooled reactor 43, i.e. preventing cooling to a temperature atwhich the synthesis catalyst is no longer sufficiently active.

The heat exchanger 31 may be used for preheating the make-up water feed57 and/or to provide heat for distillation of the crude methanol. Someor all of the water separated in separator 32 as stream J and/or amethanol/water stream separated in the distillation stage, may berecycled as part of the make-up water 57.

In the alternative methanol synthesis loop shown in FIG. 2, the heatexchange reactor 39 of FIG. 1 is replaced by a quench reactor 60 andfurther heat exchangers 61, 62 and 63 are provided to heat the feed tothe quench reactor to the desired synthesis inlet temperature. Make-upgas may be fed as stream M to the loop via line 35 where it mixes withrecycled unreacted gas (stream N) which has been heated in heatexchanger 61. Part of the resultant synthesis gas is heated in heatexchangers 62 and 63 to the desired synthesis inlet temperature and isfed as stream O via line 38 to the inlet of the synthesis reactor 60.The remainder of the synthesis gas is fed as stream O′ via line 63 tothe synthesis reactor 59 as quench gas. Typically quench gas is injectedinto the synthesis reactor 60 at a plurality of locations. The reactedgas from synthesis reactor 60 is passed via line 43 to heat exchanger 63and then may be mixed with further make-up gas supplied as stream L vialine 34 and fed to the water-cooled reactor 43. The reacted gas fromreactor 43 is cooled in heat exchangers 62 and 61 and then furthercooled in heat exchanger 47 and then fed to the separator 48. Part ofthe separated unreacted gas, stream T, is fed to the circulator 37 asrecycle gas while the remainder is taken from the loop as a purge streamU via line 51.

Part of the make-up gas may be diverted via line 65 as stream K′ andused to augment stream O′ to give the quench gas stream Z.

In FIG. 2 a further modification is shown by the region enclosed by thedotted line. Thus in order to increase further the amount of methanolformed. the purge gas stream U taken from the loop via line 51 issubjected to a further step of methanol synthesis. Thus the purge gasstream U is fed to a feed/effluent heat exchanger 66 and then to afurther heat exchanger 67 where it is heated to the desired synthesisinlet temperature. The heated purge gas is then fed as stream Q′ to afurther synthesis reactor 68 which, like reactor 43, may be a reactorcooled by pressurised water. The reacted purge gas, stream R′, is thenfed to feed/effluent heat exchanger 66 and to a cooler 69 wherein it iscooled to below the dew point of the methanol therein. The cooledreacted purge gas is then fed via line 70 to a separator 71 wherein thecondensed methanol is separated as stream S′. The residual unreacted gasstream U′ is then taken as the purge 52 while the separated methanol istaken, via line 72, and added to the condensed methanol in line 49separated in the loop separator 48. The hydrogen-rich gas added to thefeedstock via line 11 may be taken from the purge 52.

The invention is further illustrated by the following calculatedexamples in which all pressures, temperatures and flow rates (in kmol/h)have been rounded to the nearest integer.

EXAMPLE1

In this example the flow sheet follows the scheme of FIG. 1. Thefeedstock (stream A) is natural gas and the make-up water (stream W)comprises fresh water together with the condensate (stream J) separatedin separator 32 and a stream of water containing some methanol separatedin a stage of distillation of the crude methanol. In this example all ofthe make-up gas (stream K) is added as stream M to the recycledunreacted gas (stream N) from the circulator 37. The loop operates at acirculation rate of 2. In order to avoid overcooking of the catalyst inthe water-cooled reactor 43, a substantial proportion of the hot waterleaving the reactor 43 via line 53 is recyded directly as stream X. Theamount of catalyst required for the water-cooled reactor 43 is about 2½times that required in the heat exchange reactor 39.

The flow rates, temperatures and pressures of the various streams areshown in the following Table 1.

TABLE 1 P (bar Flow rate (kmol/h) Stream T (° C.) abs) CH₄ CO CO₂ H₂O H₂N₂ O₂ CH₃OH A  20 45 3409*   0  22   0  12 16 0  0 B  40 45  20   4  17  0  105  4 0  1 C 230 45 3429*   4  39   0  116 20 0  1 D 257 45  0   0  2 57652    1  0 0  33 E 450 45 3429*   4  40 7652  117 20 0  34 F 69340 2936   251  718 6083 3333 20 0  0 G 150 45  0   0   0   0   0 171669    0 H 975 40 224 2557 1125 6303 8536 37 0  0 I 528 39 224 25571125 6303 8536 37 0  0 J  40 38  0   0   2 6274   1  0 0  0 K 146 84 2242557 1123  29 8535 37 0  0 L  0  0  0   0   0   0   0  0 0  0 M 146 84224 2557 1123  29 8535 37 0  0 N  48 84 3278   706 2799  12 17486  587 0 141 O  80 84 3502  3263 3922  41 26022  624  0 141 P 265 82 3502  18573230  733 21135  624  0 2238  Q 265 82 3502  1857 3230  733 21135  624 0 2238  R 245 81 3502   751 3104  859 18545  624  0 3470  S  40 78  28  3  138  846  12  2 0 3321  T  40 78 3474   748 2967  13 18532  623  0149 U  40 78 196  42  167   1 1046 35 0  8 V 202 45  0   0   0  191   0 0 0  0 W 102 45  0   0   2 7843   1  0 0  35 X 244 45  0   0  10342101    4  0 0 198 Y 240 45  0   0  11 399753    5  0 0 231 *inaddition contains 398 kmol/h of higher hydrocarbons expressed asCH_(2.98)

The methanol in stream S, less the amount of methanol recycled from thesubsequent distillation, amounts to about 2525 tonnes per day.

EXAMPLE 2

In this example, the feedstock and conditions are the same as in Example1 except that the loop operates at a circulation rate of 1 and part(about 60%) of the make-up gas stream K by-passes the heat exchangereactor 39 and is fed as stream L and added to the effluent, stream P,from the heat exchange reactor 39. In the following Table 2, the flowrates, temperatures and pressures of the streams are shown. The amountof catalyst required in the heat exchange reactor 40 is about half thatrequired for the heat exchange reactor in Example 1 and the amount ofcatalyst required for the water-cooled reactor 44 is about 4% more thanthat required for the water-cooled reactor 44 in Example 1. Since theflow rates, temperatures and pressures of the streams, including thewater streams, in the production of the make-up gas are essentially thesame as in Example 1, they are omitted for brevity. The slightdifference in the composition of the make-up gas stream K results fromthe different composition and amount of the hydrogen-containing stream Brecycled from the synthesis loop.

TABLE 2 P (bar Flow rate (kmol/h) Stream T (° C.) abs) CH₄ CO CO₂ H₂O H₂N₂ CH₃OH A  20 45 3409*   0  22  0  12  16  0 B  40 45  9   7  23  0 108  2  1 K 146 84 220 2560 1123  29 8513  35  0 L 146 84 132 1536  674 17 5108  21  0 M 146 84  88 1024  449  12 3405  14  0 N  49 84 772  6021948  4 8952 127 74 O  75 84 860 1626 2397  16 12357  141 74 P 256 82860  937 2005 409 9800 141 1156  Q 223 82 993 2473 2679 426 14908  1621156  R 249 81 993  769 2638 467 11377  162 2901  S  40 78  12   4  164462  12  1 2807  T  40 78 980  765 2473  5 11365  161 94 U  40 78 208 162  525  1 2413  34 20 *in addition contains 398 kmol/h of higherhydrocarbons expressed as CH_(2.98)

In this example, although the methanol production is decreased comparedto Example 1, the power requirement of the circulator is only about halfthat of Example 1, and the total amount of catalyst required is about89% of that required for Example 1.

EXAMPLE 3 (COMPARATIVE)

By way of comparison, in this example, Example 1 is repeated using thesame amounts of feedstock but replacing the water-cooled reactor 43 by aheat exchanger heating the pressurised water stream 58. Because of theomission of the water-cooled reactor 43, the circulation rate isincreased to 4. The amount of catalyst required for the heat-exchangereactor 39 is about 60% more than that required for the heat exchangereactor 39 in Example 1. Since there is no catalyst cooled by the waterstream 58, there is no need to recycle part of the hot water and sostream X is omitted. In the following Table 3, the flow rates,temperatures and pressures of the streams are shown. Again since theflow rates, temperatures and pressures of the streams, including thewater streams (with the exception of stream X and consequently alsostream Y), in the production of the make-up gas are essentially the sameas in Example 1, they are omitted for brevity. The slight difference inthe composition of the make-up gas stream K again results from thedifferent composition and amount of the hydrogen-containing stream Brecycled from the synthesis loop.

TABLE 3 P (bar Flow rate (kmol/h) Stream T (° C.) abs) CH₄ CO CO₂ H₂O H₂N₂ CH₃OH A 20 45  3409*   0  22  0   12  16  0 B 40 45  20   9  13  0  95   4  1 K 146  84  225 2558 1124 29  8534  37  0 N 48 84 7054 33154497 26 33582 1275 283 O 67 84 7280 5874 5621 55 42115 1312 283 P 257 84 7280 3413 4731 945  34526 1312 3632  S 40 78  30   6  110 918    12  2 3342  T 40 78 7250 3407 4622 27 34515 1310 290 U 40 78  196  92  125 1  933  35  8 *in addition contains 398 kmol/h of higher hydrocarbonsexpressed as CH_(2.98)

The amount of methanol produced is similar to that produced in Example1, but the power requirement for the circulator 37 is about twice thatrequired in Example 1.

EXAMPLE 4

In this example, only the synthesis loop is shown and this follows theflowsheet of FIG. 2.

Make-up gas (stream K) supplied at a rate of 27987 kmol/h at about 84bar abs. and at a temperature of 116° C. is divided into three streams.One part, stream M, representing about 21% of the total, is fed to thesynthesis loop where it is mixed with recycle gas (stream N) supplied ata rate of 55000 kmol/h from circulator 37 via heat exchanger 61. Thesystem thus operates at a circulation rate of about 1.97. 25% of theresultant mixture of streams M and N is fed to heat exchangers 62 and 63where it is heated and fed, as stream O, to the inlet of a quenchsynthesis reactor 60. The remainder (stream O′) of the mixture ofrecycle gas and make-up gas streams N and M is then mixed with thesecond part (stream K′) of the make-up gas to form a quench stream Z.Stream K′ represents about 49% of the make-up gas. Stream Z is used asthe quench gas in the quench reactor 60. The quench reactor typicallyhas 5 beds of catalyst and is operated with bed exit temperaturesprogressively decreasing from 280° C. (first bed) to 260° C. (finalbed). The quench gas is introduced between each bed in such proportionsthat the temperature of the gas leaving the previous bed is decreased toa temperature in the range 215-220° C. before the mixture of reacted gasand quench gas enters the next bed. The reacted gas (stream P) leavesthe final bed at a temperature of 260° C. and at a pressure of 82 barabs. The reacted gas stream P is cooled in heat exchanger 63 and thenthe remainder, about 30%, of the total make-up gas is added as stream Lto give a gas stream Q at 245° C. which is fed to the water-cooledreactor 43. This reactor is operated to give an exit temperature of 222°C. The volume of catalyst employed in the water-cooled reactor 43 isabout 68% of that used in the quench reactor 60. The reacted gas, at apressure of 80 bar abs., is then fed as stream R to the heat exchangertrain 62, 61 and 47 wherein it is cooled to 35° C. and fed to theseparator 48. The separated crude methanol is taken as stream S whilethe separated unreacted gas (stream T) is divided into a recycle streamand a purge stream U. The recycle stream at a pressure of 80 bar abs. isfed to the circulator 37 where it is compressed to 84 bar abs and fed toheat exchanger 61 to give stream N.

The purge stream U is heated in heat exchangers 66, 67 to 220° C. andfed as stream Q′ to a synthesis reactor 68 cooled by pressurised water.The volume of catalyst in reactor 68 is about 10.5% of that used in thequench reactor 60. More methanol is synthesised in reactor 68 to give areacted purge gas stream R′ at 79 bar abs at a temperature of 221° C.The reacted purge gas stream R′ is cooled by heat exchangers 66, 69 to35° C. and fed to separator 71. The unreacted gas is taken as the purgestream U′ and the separated crude methanol stream S′ is added to thecrude methanol stream S from loop separator 49 to give a final crudemethanol product stream.

The flow rates and temperatures of the components of the streams areshown in the following Table 4.

TABLE 4 temp Flow rate (kmol/h) stream (° C.) CH₄ CO CO₂ H₂O H₂ N₂ CH₃OHK 116  952 4193 2064  46 20648  84  0 M 116  200  881  433  10  4336  18 0 N 114 5540  799 2068  20 45861 501 212 O 223 1435  420  625   7 12549130  53 O′ 114 4305 1260 1876  22 37648 389 159 K′ 116  467 2055 1011 23 10118  41  0 Z 115 4772 3314 2887  45 47765 430 159 P 260 6207 16102238 1327 52241 560 3611  L 116  286 1258  619  14  6194  25  0 Q 2456492 2868 2857 1341 58436 585 3611  R 222 6492  934 2493 1704 53478 5855908  S  35  35   2  83 1681   21  1 5661  T  35 6458  931 2410  2353457 584 247 U  35  918  132  342   3  7596  83  35 R′ 225  918  31 115  231  6710  83 364 U′  35  916  31  113   4  6709  83  25 S′  35  2   0   2  226   1  0 340 S + S′  35  37   2  85 1907   22  1 6001 

EXAMPLE 5 (comparative)

By way of comparison, Example 4 was repeated but heat exchanger 63 andwater-cooled reactor 44 are omitted and the reacted gas stream P fromquench reactor 60 is fed directly to the exchanger train 62, 61, 47. Thetotal amount of make-up gas (stream K) is decreased to 16804 kmol/h. Thesystem thus operates at a circulation ratio of 3.27. Stream M forms 25%of the total make-up gas. As in Example 4, 25% of the mixture of streamsM and N is fed to heat exchanger 62 and is fed as stream O to the inletof the quench reactor 60. The remaining 75% of the mixture of streams Mand N forms stream O′ and is mixed with the remaining 75% of the makeupgas (stream K′) to form the quench gas stream Z.

The flow rates and temperatures of the components of the streams areshown in the following Table 5.

TABLE 5 temp Flow rate (Kmol/h) stream (° C.) CH₄ CO CO₂ H₂O H₂ N₂ CH₃OHK 116  572 2517 1239  28 12397  50  0 M 116  143  629  310  7  3099  13 0 N  99 5531 1461 1665  21 45612 500 210 O 218 1419  523  494  7 12178128  53 O′ 100 4256 1568 1481  21 36534 384 158 K′ 116  429 1888  929 21  9298  38  0 Z 104 4684 3456 2411  42 45831 422 158 P 260 6103 16091871 1082  50171 550 3613  S  35  21   2  40 1059    12  1 3382  T  356082 1607 1831  23 50159 550 231 U  35  551  146  168  2  4546  50  21R′ 221  551  12  50 118  3933  50 270 U′  35  550  12  49  2  3932  50 16 S′  35   2   0   1 116   1  0 254 S + S′  35  22   2  41 1174    13 1 3636 

By comparison with Example 4 it is seen that the addition of thewater-cooled reactor 43 and addition of part of th make-up gas betweenthe quench reactor 60 enables a conventional synthesis loop to beuprated to increase the amount of methanol produced by about 65% withoutincreasing the duty of the circulator 37.

I claim:
 1. A process wherein methanol is synthesised in a loop from asynthesis gas mixture comprising hydrogen and carbon oxides in at leasttwo synthesis stages, synthesised methanol is separated and at leastpart of the unreacted synthesis gas is recycled to the first stage, andmake-up gas is added to the loop, wherein in least the final synthesisstage of the loop, the synthesis is effected in indirect heat exchangewith water under sufficient pressure to prevent boiling, whereby astream of heated pressurised water is produced, and the make-up gas isproduced by a process including catalytically reacting a hydrocarbonfeedstock with steam at an elevated temperature and at an elevatedpressure equal to, or less than, the pressure of said stream of heatedpressurised water and at least part of said steam is introduced bydirect contact of said hydrocarbon feedstock with said stream of heatedpressurised water.
 2. A process according to claim 1 wherein some or allof the make-up gas is added to the recycled unreacted gas after thefirst synthesis stage.
 3. A process according to claim 1 wherein atleast 5% of the make-up gas is added to the recycled unreacted gasbefore the latter is fed to the first synthesis stage.
 4. A processaccording to claim 1 wherein the first synthesis stage is effected in aquench reactor.
 5. A process according to claim 1 wherein the firstsynthesis stage is effected in a heat-exchange reactor having a bed ofsynthesis catalyst with a plurality of cooling tubes extendingtherethrough and the synthesis gas is fed to the tubes and is heated byindirect heat exchange with the synthesis gas passing through the bed ofcatalyst.
 6. A process according to claim 5 wherein 30-90% of themake-up gas is added to the recycled unreacted gas before the latter isfed to the first synthesis stage.
 7. A process according to claim 1wherein the circulation ratio is in the range 1 to 3.